I had concerns about my daughter not crawling either. Give it plenty of time. She is 8 1/2 months old now and just started the military crawl. Because of our concerns we decided to address it with our doctor. The doctor said there is such a wide range for babies to develop and most will do so at their own pace.
I know how you feel, but I can assure you that I feel better now that we've addressed it with the doctor.